Pulmonary embolism (PE) is a blockage of the main artery of the lung or one of its branches by displaced clot or debris that has traveled from elsewhere in the body through the bloodstream. PE is a potentially life threatening condition with an annual incidence of approximately 600,000 cases. Prompt diagnosis and treatment of PE can dramatically reduce the morbidity and mortality associated with the condition. First-line therapy for PE is anticoagulation. For those patients in who anticoagulation is ineffective or inappropriate, intravascular filters can be deployed. These filters are designed to capture potentially fatal clots or debris from traveling into the lungs, most commonly from pelvic or lower extremity veins, or less commonly from the upper extremity veins. As such, filters are placed most commonly in the inferior vena cava (IVC) or superior vena cava (SVC), respectively. In many instances, the filter is placed permanently in the body. However, in those clinical circumstances where the risk of thromboembolism is impermanent, the temporary placement and subsequent retrieval of the filter may be preferable.
Generally, the quality of a filter is measured by at least some of the following characteristics: effectiveness in capturing blood clots without disrupting the normal flow of blood within the vessel, reliability in being deployed at a predictable location within the desired vessel, ability to remain in the deployed location without migration, ability to deploy and retrieve with minimal trauma to the vessel, structural integrity to allow for permanent deployment when desired, and allowing for a small deployment and retrieval system. Although multiple filter designs have been proposed, the difficulty in achieving these potentially conflicting objectives allows for improvement to the current designs. In particular, the objectives of having the filter securely anchored to the wall while also being readily detachable at any time have been difficult to assimilate into an effective design. Many prior art retrievable filters require that the removal force be in a vector parallel to the longitudinal axis of the blood vessel. At the same time, the filter must resist displacement in this same direction, to prevent migration into less optimal locations, including the heart and lung. Some filters attempt to minimize engagement with the blood vessel in order to facilitate subsequent removal. But this requires compromise in other aspects of the ideal filter, for example resistance to migration. Other designs, conceding the difficulty in achieving these disparate goals, abandon the concept of a fully retrievable filter and rely on a permanently embedded portion to optimize anchoring and centering of the filter. U.S. Pat. No. 6,506,205 is an example of such a design.
The design of the present invention improves on shortcomings in the prior art filter devices by altering the force necessary for retrieval from a longitudinal force to a predominantly rotational force. The disparate objectives of resistance to migration in the longitudinal direction, and extraction by a longitudinal force, are therefore ameliorated. In addition, altering the force required for retrieval can allow for retrieval of the filter from an upstream (e.g. femoral vein) approach. The design allows for secure anchoring, a self-centering conical filter element, with minimal vessel trauma during deployment and extraction and structural integrity to allow for permanent implantation when desired.